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We studied the expression of myosin heavy chain (MHC) isoforms, utilizing electrophoretic methods, in rib cage (RC) muscles: the scalenus medius, the parasternal, cephalic, midthoracic, and caudal intercostal muscles; and in the diaphragm (DI) of rats during postnatal development and when mature. At day 1, all RC muscles and the DI expressed MHC neonatal/embryonic (69-92% of total MHC complement) with little MHC slow and 2A; the RC muscles alone expressed a small proportion of MHC 2B (2-4%). On day 4, MHC neonatal/embryonic expression still predominated (55-71%) but increased MHC 2A expression was observed in both the RC (11-21%) and DI (31%); MHC 2B (5-7%) was noted in the RC muscles but not the DI. By day 14, MHC neonatal/embryonic and 2A expression each comprised a third of the total MHC complement of the RC muscles, MHC 2X was first observed, and MHC 2B expression increased. The day 14 DI was comprised of equal proportions of MHC neonatal/embryonic, slow and 2A with little MHC 2X (11%). The adult and day 30 animals expressed comparable muscle-specific MHC phenotypes: the DI characterized by a proportional mixture of MHC slow, MHC 2A, and MHC 2X, with little MHC 2B, whereas the RC muscles expressed predominantly MHC 2B (40-62%). We conclude that the RC muscles and DI show comparable MHC phenotypes in the immediate newborn period but differ in their MHC expression during postnatal development and when mature. The RC muscles show only minor intermuscle variations in MHC phenotype during development, and when mature are characterized by fast MHC isoform expression, particularly MHC 2B.